linux-kernel - Re: [PATCH v2 1/2] selftests/sgx: Use rip relative addressing for encl

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Message-ID: <cc2c191065c489eb22bb15bf3dd2e3cddc822543.camel@kernel.org>
Date:   Thu, 31 Mar 2022 01:30:08 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Shuah Khan <skhan@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        "open list:INTEL SGX" <linux-sgx@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] selftests/sgx: Use rip relative addressing for
 encl_stack

On Wed, 2022-03-30 at 14:29 -0700, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 3/30/2022 1:40 PM, Jarkko Sakkinen wrote:
> > On Wed, 2022-03-30 at 13:05 -0700, Reinette Chatre wrote:
> > > 
> > > 
> > > On 3/30/2022 12:03 PM, Jarkko Sakkinen wrote:
> > > > On Wed, 2022-03-30 at 10:40 -0700, Reinette Chatre wrote:
> > > > > Could you please elaborate how the compiler will fix it up?
> > > > 
> > > > Sure.
> > > > 
> > > > Here's the disassembly of the RBX version:
> > > > 
> > > > [0x000021a9]> pi 1
> > > > lea rax, [rbx + loc.encl_stack]
> > > > 
> > > > Here's the same with s/RBX/RIP/:
> > > > 
> > > > [0x000021a9]> pi 5
> > > > lea rax, loc.encl_stack
> > > > 
> > > > Compiler will substitute correct offset relative to the RIP,
> > > > well, because it can and it makes sense.
> > > 
> > > It does not make sense to me because, as proven with my test,
> > > the two threads end up sharing the same stack memory.
> > 
> > I see, I need to correct my patch, thanks!
> > 
> > RBX gives correct results because of the binary organization,
> > i.e. TCS's are placed to zero offset and forward, and 
> > unrelocated symbol is just compiled in as an untranslated
> > offset.
> > 
> > RPI is given correct results but how the semantics work
> > right now is incompatible.
> > 
> > Still, even for kselftest, I would consider a switch
> > because that way:
> > 
> > 1. You can layout binary however you wan and things
> >    won't break.
> > 2. You can point to any symbol not just stack, if
> >    ever need.
> >    
> > I admit it works semantically but it just super
> > unrobust.
> 
> I do not think that we need an exceptionally flexible
> runtime as part of the SGX selftests but instead something
> that is easy(*) to understand while also sufficient to support
> the tests.
> 
> Reinette
> 
> * I do not actually consider the existing enclave test binary
>   easy to understand (this thread is proof) but keeping its
>   complexity to be minimal would benefit folks needing to
>   ramp up on SGX and/or debug kselftest failures.

Based on you feedback I refined the patch:

https://lore.kernel.org/linux-sgx/20220330222834.139769-1-jarkko@kernel.org/T/#u

BR, Jarkko



	
	




BR, Jarkko